Glass sheet processing systems such as the type disclosed by U.S. Pat. No. 4,575,390 include bending apparatus having one or more molds adapted to be positioned within a heating chamber and receive a heated glass sheet from a roller conveyor in preparation for tempering and/or bending.
Briefly, the glass processing system typically includes a furnace defining a heating chamber through which glass sheets are conveyed for heating in preparation for bending. The bending apparatus of the preferred system includes a roller conveyor for supplying heated glass to one or more curved molds. The curved molds typically take the shape of a surface having a complex curvature that is generally convex in nature or a complimentary concave surface in the form of an open center ring. The heated glass is formed by placing the sheet in a series of steps on a mold and moving the mold(s) relative to the glass to provide an accurately formed curvature according to a preselected design. The molds are each typically mounted for movement along a single axis. Thus, a position controller of the present invention would be required for each of these movable members.
The quick and accurate positioning of the molds during various stages of the process is an important factor in achieving a high quality product in this bending and tempering process. Thus, the processing system must include a central control system capable of simultaneously monitoring various conditions throughout the system and simultaneously positioning various movable components of the system according to the process.
The central control system typically includes an operator interface or console which may be in the form of a teletype unit for inputting various data, such as selected important mold positions and desired temperatures, into a master computer. The master computer monitors various conditions, such as the actual temperature at various selected points in the furnace, and transmits this information to the operator through the console or other suitable data output device.
The master computer also communicates with one or more position controllers. The position controller processes positioning commands received from the master computer, receives input from a position sensor located on the driven component, and issues a signal to activate the variable speed drive unit for that component causing the driven component to move according to the appropriate velocity profile to the desired point.
One disadvantage of existing glass processing position control systems is that communication between the master computer and the slave computer is limited to two-wire open-loop transmission of single ASCII character commands. This limited communication, while sufficient to allow for transmission by the master computer of single character motion commands, and transmission by the slave computer of single character acknowledgement commands, greatly restricts the flexibility of the system.
For example, the master computer cannot receive actual position information for any components from the slave computer. Thus, if the operator has manually moved (jogged) any of the components to a different position, the slave controller can ascertain this new position via its communication with a position encoder, but the master computer is not updated accordingly. Similarly, if it is desirable that a component move to a selected end point and then oscillate between two points from this position for a selected period of time, the master computer does not track the location of the mold at all times during this oscillation routine.
Also, as a result of the limited communication capabilities between the master and slave controller, the positions and desired velocity profiles for the end points cannot be downloaded by the master computer. The locations of preselected points ("end points"), along with the drive parameters and move characteristics for those points are permanently "burned" into an Electrically Programmable Read-Only Memory (EPROM) and cannot be downloaded or otherwise changed from the master computer.
Another disadvantage of existing systems is that, in situations where a movable component is periodically shuttled into and out of a furnace during the glass processing cycle, the shuttle upon which the component is mounted often undergoes thermal expansion or contraction. Thus, the actual location of the component changes during processing. This uncontrollable thermal expansion/contraction causes positioning problems, particularly when the component is programmed to move to a selected end point wherein accurate positioning is important, such as where one mold is to be mated with another mold. The operator is thus forced to make any compensation for this thermal expansion or contraction on the basis of his observation of the change in position of the mold.
Another disadvantage of present control systems is that the master computer board is different in configuration from the slave controller board so that separate replacement boards for each of the controllers needs to be kept in stock. Also, though the slave controller boards for each of the different movable molds is identical, separate replacement ROM chips, each corresponding to a particular mold or other movable component, must be kept in stock.